1. Field of the Invention
The present invention relates generally to a system for measuring turbulence in fluid flow and more particularly to such a system that measures momentum flux caused by turbulent fluctuations.
There are two fundamentally different types of flow -- laminar and turbulent. Turbulent flow is much more common in nature and in engineering devices than laminar flow. For example, the flow in rivers and the motion of the air in the atmosphere are practically always turbulent. The fluid motions with which the engineer is concerned are turbulent in most cases. In turbulent motion the velocity and pressure at a fixed point do not remain constant with time but perform irregular fluctuations of high frequency. In describing a turbulent flow in mathematical terms, it is convenient to separate it into a mean motion and a fluctuating or eddying motion. In turbulent flow the fluctuations influence the mean motion in such a manner that the latter exhibits an apparent increase in the resistance to deformation. Stated another way, the presence of fluctuations manifests itself in an apparent increase in the viscosity of the fundamental flow.
The flow adjacent to the surface of a body moving in a fluid is called the boundary layer. The flow in this layer may be laminar at low Reynolds numbers ##EQU1## AND MAY BECOME TURBULENT WHEN THE Reynolds number exceeds a certain critical value. This change has a favorable consequence because the violent intermingling of particles enables the turbulent layer to stick to the surface better than does the laminar layer, which contains less kinetic energy and leaves the surface earlier. At low Reynolds numbers, especially in the range where the drag coefficient of a sphere or cylinder is almost constant and has the larger value, the boundary layer is laminar and the early separation of the flow creates a broad wake filled by vortices. Then, at a certain high Reynolds number, the flow in the boundary layer becomes turbulent, the separation is delayed, and the size of the wake is reduced.
In many fields, and especially in the field of aeronautics, it is important to be able to measure the transfer of momentum flux due to turbulent fluctuations. In aircraft design, the Navier-Stokes equations are available tools which may be employed to solve for the drag and lift characteristics of a given airfoil in turbulent flow. These equations cannot be solved unless the time-averaged momentum fluxes associated with turbulent flow are known. No apparatus is known in the prior art which will measure in real time instantaneous momentum flux (Reynolds stress wave) and time-averaged momentum flux (Reynolds stress).
At present, the standard instrument for measuring air velocity is the hot-wire anemometer. The hot-wire anemometer is a resistive flow-velocity transducer which consists essentially of a thin heated wire supported at its ends so that it loses heat to the air stream which is being measured. This convective heat loss varies approximately with the square root of fluid velocity. Two operating modes are used for the hot-wire anemometer. In both modes the wire is heated by the current flowing through it. When the wire is operated at constant current, its resistance increases with cooling and the resulting bridge unbalance produces an output voltage which can be related to fluid velocity. Faster response time is obtained by operating the transducer in a constant-temperature mode. Sometimes two-hot-wire anemometers are oriented in a cross configuration to obtain "directional" information. This arrangement will only work if the flow is planar (in the plane of the two wires). Obviously, any turbulence transverse or oblique to the plane of the wires will cause cooling of the wires and will generate errors. The wire in a hot-wire anemometer cannot discern whether air flows are approaching from the side, top, bottom, or any other direction.
In addition to the above disadvantage, the hot-wire anemometer is unsatisfactory because it is influenced by parameters of the fluid that is to be measured (density, temperature, chemical composition).
Another prior art device is based on the principle of measuring the doppler shifts of light scattered from small suspended particles in the flowing stream. However, the data obtained from these measurements is that of a spectrum of mainstream and cross velocities of a particular particle as a function of time and accordingly the data must be tediously correlated to be useful. Moreover, this technique is further dependent upon the density fluctuation of the fluid.
In yet another device for measuring the flow unsteadiness in a turbulent fluid flow, a device comprising a lift-sensor element having a lift-sensing surface has been developed. This device senses flow velocity disturbances perpendicular to a mainstream fluid flow and develops a time varying lift force with corresponding displacement responsive to time variation of the velocity disturbances. Thus, this probe is limited in that it detects only the perturbed velocity vectors in other than the mainstream direction and thus cannot be used to provide measurements of drag forces in the fluid. In addition, the probe is sensitive to the positioning of the lift-sensor and hence, its indications are dependent upon the rate of change of the angle of attack of the fluid.
Examples of prior art fluid velocity and/or direction sensing instruments are found in U.S. Pat. No. 3,696,673, "Methods and Means of Measuring Velocity Fluctuations in Unsteady Flow," Ribner et al.; U.S. Pat. No. 3,217,536, "Force Vector Transducer," Motsinger et al.; U.S. Pat. No. 3,552,204, "Means for Detecting and Recording Water Wave Direction," Tourmen; U.S. Pat. No. 2,985,014, "Anemometer," Doersam, Jr.; U.S. Pat. No. 3,264,869, "Process and Apparatus for Studying Currents," Erdely; and U.S. Pat. No. 3,695,103, "Current and Turbulence Meter," Olson. None of these patents reveal instruments capable of measuring the transfer of momentum flux resulting from turbulent flow.